Radio Frequency (RF) amplifiers are used in a variety of devices, including mobile communications devices, such as mobile telephones. In particular, an RF power amplifier is employed to amplify and transmit an RF signal from a mobile communication device.
The data to be transmitted from the mobile communication device may be impressed upon the transmitted RF signal by employing any one of a number of modulation techniques. Some of the techniques that are employed in contemporary communication devices produce transmitted signals with an RF amplitude envelope that varies with time. Some examples of transmission protocols that produce transmitted signals with an RF amplitude envelope that varies with time include Code Division Multiple Access (“CDMA”), Wideband Code Division Multiple Access (“WCDMA”), and Orthogonal Frequency Division Multiplexing (“OFDM”).
FIG. 1 shows an exemplary RF power amplifier 100 that may be employed in a mobile communication device operating with a CDMA protocol. RF power amplifier 100 includes gain element 110 and RF impedance transformer 120. RF power amplifier 100 further includes input DC blocking element 115, RF blocking element 125, and output DC blocking element 135. In general, gain element 110 may comprise an RF power transistor such as a field effect transistor (FET) or bipolar junction transistor (BJT), biased as appropriate. In general, RF impedance transformer 120 is an approximate loss-less network of fixed-value capacitive and/or inductive elements for matching an output impedance of gain element 110 to an impedance of a load 40 (e.g., an antenna). Input and output DC blocking elements 115, 135 are in most cases capacitors, while RF blocking element 125 in most cases is an inductor or transmission line presenting a high impedance at the operating RF frequencies.
In operation, an RF input signal is supplied through input DC blocking element 115 to a node 150 which corresponds to an input of gain element 110. DC current is also supplied to gain element 110 from a DC power source (e.g., a battery) 20 though RF blocking element 125. Gain element 110 amplifies the RF input signal and outputs an amplified signal at node 175. The amplified signal is passed by RF impedance transformer 120 as an RF output signal to load 40 (e.g., an antenna).
Meanwhile, at node 175 the average voltage is constrained to be the voltage VCC of the DC power source. Also, at node 175 the instantaneous voltage will be constrained by the operating requirements of gain element 110. For example, if gain element 110 is a bipolar transistor connected in common-emitter configuration, the instantaneous voltage at node 175 will be constrained to be greater than zero. Furthermore, gain element 110 is in general required to provide a certain RF output power to load 40. The RF output power delivered by gain element 110 at node 175 is:Power=REAL{(VRF*IRF*)2}  (1)where VRF is the complex RF voltage at node 175, and IRF is the complex RF current.
Also, the RF impedance at node 175 looking toward load 40 is:ZG=VRF/IRF   (2)Also the impedance of load 40 is in general selected for convenience to be something in the range of 50 ohms to 75 ohms. Meanwhile, the output impedance of gain element 110 must be low enough to enable RF power amplifier 100 to produce a required RF output power level, consistent with the aforementioned constraints on voltages at node 175.
Accordingly, RF impedance transformer 120 is employed to match the impedance of load 40 to the required output impedance of gain element 110. In particular, as discussed above, RF impedance transformer 120 is an approximate loss-less network of fixed-value capacitive and/or inductive elements selected for maximizing the power transfer from gain element 110 to load 40. That is, RF impedance transformer 120 is, to a great approximation, a linear time invariant (LTI) network.
Meanwhile, an RF power amplifier that transmits the output signal from a mobile communication device often represents the largest power drain on the mobile power supply (e.g., a battery). Also, the required time between battery charges is often shorter than desired. The required time between charges can be lengthened if the efficiency of the RF power amplifier could be improved. Furthermore, in other applications of RF amplifiers where the transmitted power is very large, such as in television broadcast transmitter that operates 25 hours/day, 365 days/year, the electricity costs can become significant. These costs can be reduced if the efficiency of the RF transmitter amplifier can be improved.
Turning again to FIG. 1, for a particular value of transformed RF load impedance at node 175, and for a particular battery voltage VCC, there will be one RF output power level for which RF amplifier 100 converts DC (e.g., battery) energy into RF transmit energy with maximum efficiency. Because the maximum energy conversion efficiency occurs at only one RF output level, signals which have a time-varying RF amplitude envelope (e.g., CDMA; WCDMA) will almost always cause RF amplifier 100 to operate at less than peak, or maximum, energy conversion efficiency.
Meanwhile, for RF amplifier 100, maximum energy conversion efficiency almost always occurs at an RF output power level that is higher than the RF output level where the RF power gain is at a maximum. That is, maximum energy conversion efficiency occurs when RF amplifier 100 is in significant gain compression. However, when CDMA or WCDMA signals (and many other signals) are applied to RF amplifier 100 in a state where it is gain compression, then the signals are distorted in a non-linear way, causing their spectral widths to grow substantially. Meanwhile, there are specifications which limit the spectral occupancy of these signals to prevent interference. Accordingly, to avoid the spectral expansion that occurs by the gain compression at or near the point of maximum power conversion efficiency, when RF amplifier 100 amplifies such signals, it is operated at a point well below maximum energy conversion efficiency.
The result is that RF amplifier 100 and similar amplifiers operating with CDMA and WDMA, and similar signals, are operating at far from maximum efficiency in converting DC power to RF transmitted power. As explained above, in a mobile device this can be a significant limitation on the battery life, and in very high power transmit applications, it can increase the operating (electricity) costs.
What is needed, therefore, is an RF amplifier with improved power conversion efficiency. What is also needed is an RF amplifier that can provide improved power conversion efficiency when operating with signals having time-varying RF amplitude envelopes. What is further needed is an RF amplifier that can provide improved power conversion efficiency when operating with signals having time-varying RF amplitude envelopes without causing the signal spectral bandwidth to expand beyond an acceptable level.